In the unlikely case of total engine and APU failure and battery failure, the A320 is equipped with a Ram air turbine which pops out from the aircraft belly and generates electricity by spinning from the moving air stream. This is essentially using the aircraft’s momentum to generate electricity, or essentially a mini wind turbine. On an A320 it looks like this:
It automatically deploys when there is no sufficient power on board aircraft, and it basically drops mostly by a spring-load mechanism and gravity. This is notably how the famous US Airways Flight 1549 that ditched in the Hudson River managed to remain controllable.
Also, aircraft are designed to run in an extreme low-power mode in such circumstances: no in-flight entertainment, no lighting, no ventilation, and so on. The ram air turbine produces just enough power to run basic navigation, a very basic fly by wire, and limited control of a few flight surfaces, but it will be enough to get the aircraft through an emergency landing.
It’s worth noting that this system is not unique to A320 aircraft and that most commercial aircraft are equipped with a ram air turbine.
I haven’t flown A320 series in awhile now so I’m not sure about the latest versions of the airplane.
Indeed there’s a RAT (Ram Air Turbine) which drives one of the hydraulic systems which in turn drives an emergency generator. But let’s say this one failed too.
And then on top of that the last resort, the batteries, fail too.
You can still fly in “mechanical backup” mode then. You don’t control the electrical actuators of elevators and ailerons then but you fly the airplane with a mechanical link through rudders and stabilizer.
Of course that’s easier said than done and I have tried it in the simulator. It takes a while to get used to and stabilize the flight path but once that’s under control you have time to start trying resets and maybe get the engines or APU (auxiliary power unit) back. If you don’t get anything back, you’re still not crashing since you can control the glide this way.
You are then flying the pitch of the airplane through the stabilizer and you control the turns and directions with the rudder and possibly differential power if somehow magically the engines are working but their generators aren’t.
All provided you have hydraulics of course. (If you lose all hydraulics, there’s no way to control the plane anymore but that’s just like any conventional non fly-by-wire aircraft of significant size.)
This mechanical backup gives a non electrical means of controlling actuators that hydraulically control the rudder and stabilizer.
On later versions this has been replaced with a BCM (the Backup Control Module computer):
(what follows is from an older A330 manual but it’s similar on an A320:)
The BCM computer provides yaw damping, and direct rudder command with pedals, via an
independent unit, in cases of:
— Total electrical failure, or
— Loss of rudder control due to a Flight Control Computer (PRIM and SEC) failure.
— Its own electrical generator, referred to as the Backup Power Supply (BPS), which is supplied
by the B or Y hydraulic system;
— Its own sensors (gyrometers and pedals deflection);
— Control of the B and Y hydraulic actuators.
When activated, as in yaw alternate law, there is no turn coordination.
(And similarly from an older A330 manual but the same on an A320:)
Mechanical control by cable of the THS [Trimmable Horizontal Stabilizer, self written for clarifying the abbreviation] is available from the pitch trim wheel at any time,
provided B or Y hydraulic is available.
Mechanical control from the pitch trim wheel has priority over electrical control.
If you still get no electricity back in this mechanical backup mode, you can try to land the plane in this mode. However, it’s very likely that will be more of a controlled crash than a landing.
Note in the schematic here of an A320’s control system in pitch the mechanical trim. No electrics need to work for this mechanical link:
And here’s an A330 schematic of the yaw control but it’s similar on an A320. Note the rudder pedals have a mechanical link – so no electricity needed – to control the rudder.
In an Airbus A320 the Rudder and the THS have mechanical cable connections for their operation, but these cables run only till the hydraulic servos that in turn move these surfaces with the help of hydraulic pressure.
Its difficult to control an aircraft who has lost all its hydraulics systems.
As far as I know the Boeing 737 can be controlled even without Hydraulics.
Now, as you’re saying that, if all of the electrical systems failed, then what would happen? Right? (failure of APU and Batteries too)
So, as Joseph said earlier, Airbus A380 is equipped with a Ram Air Turbine (known as RAT) as shown below-
Not only in A320, you can find this system in other Aircrafts too!
Working of RAT-
A ram air turbine is a small turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.
This system can generate the amount of electricity which would be enough for lighting up navigation systems and helps pilots by making a chance for an emergency landing.
We all know about the incident that took place in Hudson River, few years back.US Airways Flight 1549, an Airbus A320 made an unpowered emergency water landing in the Hudson River after multiple bird strikes caused both jet engines to fail.
So, this system helps in emergency landing of the plane and saves many precious lives!